Exhaust gas recirculation (EGR) is a technique commonly used for controlling the generation of undesirable pollutant gases and particulate matter in the operation of internal combustion engines. This technique has proven particularly useful in internal combustion engines used in motor vehicles such as passenger cars, light duty trucks, and other on-road motor equipment. The EGR technique primarily involves the recirculation of exhaust gas by-products from the combustion process into the intake air supply of the internal combustion engine. The exhaust gas reintroduced to the engine cylinders acts to reduce the concentration of oxygen therein, which in turn lowers the maximum combustion temperature within the cylinder and slows the chemical reaction of the combustion process, decreasing the formation of nitrous oxides or NOx.
When utilizing EGR in a turbocharged diesel engine, the exhaust gas to be recirculated is typically removed upstream of the exhaust gas driven turbine associated with the turbocharger. For example, in many EGR applications the exhaust gas is diverted directly from the exhaust manifold and diverted via an EGR conduit to the intake system. Likewise, the recirculated exhaust gas is preferably re-introduced to the intake air stream downstream of the compressor and inter-cooler or air-to-air aftercooler. Reintroducing the exhaust gas downstream of the compressor and intake air cooler device is preferred due to the reliability and maintainability concerns that arise should the exhaust gas be passed through the compressor and/or intake air cooler.
At many engine operating conditions within a turbocharged diesel engine, there is a pressure differential between the intake manifold and the exhaust manifold which essentially prevents many such simple EGR systems from being utilized. For example, at low speed and/or high load operating conditions in a turbocharged engine, the exhaust gas does not readily flow from the exhaust manifold to the intake manifold. Therefore many EGR systems include an EGR driver such as a Roots-type blower or an auxiliary compressor to force the exhaust gas from the exhaust manifold to the higher pressure intake manifold. See U.S. Pat. No. 5,657,630 (Kjemtrup et al.) as merely one example of the many EGR systems that utilize a pump or blower type arrangement to drive the EGR from the exhaust manifold to the intake system. See also European Patent Application No. EP 0 889 226 A2 as well as PCT patent document WO 98/39563 that disclose the use of an auxiliary compressor wheel driven by the exhaust gas driven turbine associated with the turbocharged diesel engine. The auxiliary compressor wheel forcibly drives the recirculated exhaust gas from the exhaust manifold to the intake system at nearly all engine operating conditions.
One apparent problem with such forced EGR systems that utilize an auxiliary compressor is that the auxiliary compressor chokes long before the EGR flow requirements are at met at many light load operating conditions. Such light load operating conditions yield conditions where the exhaust manifold pressure is higher than the intake manifold pressure and the auxiliary compressor, blower, pump or other EGR driver is more of a flow restriction than an assist.
In addition, the reliability and durability of such conventional EGR systems that utilize such EGR driver means is suspect due to the failures attributable to the EGR driver components that are not sized properly to cover all operating conditions. What is needed therefor is a simple and inexpensive improvement to such forced EGR systems that improves the overall EGR system performance while minimizing the likelihood of EGR driver failures. Such an improvement or improved technique should be operable over the entire operating regime or operating conditions for such turbocharged diesel engine. In other words, the simple improvement and improved technique should further be operable over the entire range of engine speeds, loads, and inlet or exhaust temperatures and pressures.
One apparent problem with such forced EGR systems that utilize an auxiliary compressor is that the auxiliary compressor requires some flow of air therethrough to minimize the probability of compressor surge condition. A surge condition may cause premature failure to the auxiliary compressor wheel and possibly to the intake air compressor wheel, if one is used. Thus, when the EGR flow is restricted, many such forced EGR systems are prone to early failure. In other words, the reliability and durability of such conventional EGR systems that utilize such auxiliary compressor is suspect due to the failures attributable to the auxiliary compressor components that are not sized properly to cover all operating conditions, and in particular where EGR is off. What is needed therefor, is a simple and inexpensive improvement to such forced EGR systems that improves the overall EGR system performance while minimizing the likelihood of EGR driver failures. Such an improvement or improved technique should be operable over the entire operating regime or operating conditions for such turbocharged diesel engine. More importantly, what are needed are improvements to such existing EGR systems that provide reliable and durable designs of an EGR system. The present invention is directed at overcoming one or more of the problems set forth above.